Normal flora of the healthy human host

Normal flora of the healthy human host

The human body is inhabited by a large number of microorganisms, which together are called the body’s normal flora. Most members of the normal flora are bacteria, but fungi and protozoa inhabit the body as well. Some of the microbes occur on the skin, but most live on the inner surfaces of the body. These surfaces include the mucous membranes that line nose, mouth, upper respiratory tract, intestinal tract, and genitourinary tract. It is important to recognize that the internal organs and tissues of the healthy human body are usually free of microorganisms. Indeed, finding microorganisms in the internal regions of the body for instance, in the bloodstream usually means that the person is infected and ill.

The term normal flora implies that such microbial inhabitants are harmless; for the most part, normal flora microorganisms do not cause disease. They are residents of the healthy human body and are permanent occupants. In contrast, those are transients of the healthy human body may establish themselves briefly, but tend to be excluded by competition from residents or by the host’s immune defense mechanisms. Transients are not considered part of the normal flora.

Most normal flora microbes are comments: they benefit from the association with the host, but the host is not adversely affected. Others have a mutualistic association with the host they benefit the host in some fashion while thriving in the host’s body (i.e., both benefit from the association).

Some normal flora organisms can be opportunistic pathogens, which cause infections if tissue injury occurs at specific body sites, or if the resistance of the body to infection is decreased. These organisms have become especially important in recent years because there has been a rising incidence of infection from them, due to the prolonged use of antibiotics and the use of immune-suppressive chemicals for treatment.

Microbiologists study the human normal flora because it is important to know what kinds of microorganism are harbored by the healthy body. This knowledge gives insight into the kinds of infections that might occur following tissue trauma at various sites. For instance, the cause of heart valve infection may be traced to a tooth extraction. This enables the physician to evaluate the nature of the infection and treat it more effectively. Knowledge of the normal flora also leads to an understanding of overgrowth of normally absent microorganisms that may take place at a particular site. An example is a heavy growth of a yeast such as Candida albicans in the oral cavity as a consequence of intensive, broad-spectrum antibacterial drug therapy or profound suppression of the immune system. It also gives us a better appreciation of the role played by the normal flora in preventing exogenous microbes from successfully colonizing our bodies.

Origin of the normal flora

Before birth, a healthy human fetus is free of microorganisms. Under natural circumstances, the newborn infant first encounters microorganisms while passing down the mother’s birth canal, especially the vagina. It acquires them by surface contact, swallowing, or inhaling. These microbes are soon joined by others from sources in the newborn’s immediate surroundings, including the skin (such as that of the hands and breasts) and respiratory tract of those caring for the infant. During the first few days of life, the flora includes microorganisms that can grow on particular sites in the absence of competitors. Soon, as the infant is exposed to the full range of microorganisms in the immediate environment, those microbes that compete best with others in particular sites become predominant. Such microbes form a stable normal flora.

Each part of the human body, with its special environmental conditions, has its own particular mixture of microorganisms. For example, the oral cavity acquires a different natural microbial population from that of the intestines. In a short time, the child will have the same general kind of normal flora as an adult person in the same environment, the nature of that flora depends on such factors as the frequency of washing, diet, hygienic practices, and living conditions.

Effect of the normal flora on the human host

What effect does the establishment of the normal have on the body? Three approaches have been used to answer this question:

1. Use of germfree animals. If the colonization of experimental animals by microorganisms can be prevented, one can compare the properties of such germ-free animals with those of normal animals. The results can be helpful in understanding the functions of the normal flora of humans.
2. Use of antimicrobial agents. If the balance that occurs between the normal flora and the human host is altered by use of antimicrobial agents, the effects are useful in understanding the role of the normal flora.
3. Knowledge of certain characteristics of the normal flora that inhibit the growth of other microbes. These characteristics suggest that normal flora organisms may help to discourage the growth of microorganisms that are not part of the indigenous flora.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.